Grind controls water's access to coffee
Grinding increases the surface area that water can reach. A fine grind offers more surface and creates more resistance to water flow; a coarse grind offers less surface and lets water move more easily. The right size therefore depends on the method, contact time and the way water travels through or around the coffee.
There is no universal grinder number. Two models may produce different sizes at the same setting, and every coffee creates a different amount of fine particles. Visual comparisons with salt, sand or sugar are only rough guides. Brewing and tasting confirm the right point.
Starting points by brewing method
Espresso usually needs a fine grind because pressurised water crosses the coffee in a short time. AeroPress accepts a broad range, from medium-fine in short recipes to coarser in long immersions. V60 often starts around medium, while Chemex commonly benefits from medium-coarse because of its filter and larger brew volumes.
French press normally uses a coarse grind for its long immersion and cleaner separation when the plunger is lowered. Kalita and Origami often sit in the middle, although their geometry, filters and recipe change the flow. Clever combines immersion with filtration, making the medium to medium-coarse range relatively forgiving.
Knowing which way to move the grinder
If water passes too quickly and the cup tastes sour, thin or short, try finer. If the filter stalls, brewing takes far too long and dryness or roughness appears, try coarser. In immersion methods, where water does not depend so heavily on passing through a bed, time and agitation carry more weight, yet grind still changes the speed of extraction.
With espresso, also watch uniformity. An irregular flow, side jets or a drink that tastes sour and bitter at once can indicate channelling rather than a simple grind error. Even distribution and a consistent dose are essential before blaming the grinder setting.
A mental chart more useful than one number
As an initial map, methods can be ordered from finer to coarser: espresso; short AeroPress recipes; V60, Kalita and Origami; Clever and Chemex; French press. It is not a law. Dose, filter, roast, brew size, grinder and desired result can move every method along the scale.
When opening a new bag, keep the rest of the recipe stable, adjust the grinder gradually and record the result. Comparing three brews will teach you more about that coffee than trying to find a supposedly perfect setting before you begin.